Grid-Forming vs. Grid-Following: The Fundamental Difference Reshaping Power Systems

When ERCOT frequency hit 59.302 Hz during the 2021 winter storm, operators learned what happens when grid inertia disappears—and why the future belongs to inverters that can create their own voltage

At 1:55 a.m. on February 15, 2021, ERCOT operators watched their control room displays with growing alarm as system frequency plummeted to 59.302 Hz—just 0.098 Hz from triggering automatic load shedding that would have blacked out millions. The system stayed below the critical 59.4 Hz threshold for 4 minutes and 23 seconds, forcing operators to manually shed 20,000 MW of load in the largest controlled load shedding event in U.S. history.

The root cause wasn't just frozen generation equipment—it was the loss of grid inertia that normally cushions frequency excursions. As traditional synchronous generators tripped offline, the grid lost the spinning masses that provide natural frequency stability. What remained was a collection of grid-following inverters that could inject power but couldn't stabilize the system when things went wrong.

This crisis crystallized a fundamental challenge facing power system engineers: as renewable energy replaces traditional generation, who provides the grid stability services that keep the lights on? The answer is emerging in the form of grid-forming inverters—a technology that's not just another incremental improvement but represents a paradigm shift from generator-dominated to power electronics-dominated grids.

Understanding the Fundamental Difference

Grid-Following Inverters: The Current Standard

How They Work: Grid-following (or grid-tied) inverters have dominated renewable energy deployment for the past two decades. They operate as controlled current sources that fundamentally depend on the existing grid for stability:

• Phase-Locked Loop (PLL) Dependency: Continuously synchronize to grid voltage and frequency

• Current Source Operation: Inject controlled real and reactive power based on external commands

• Grid Strength Requirements: Require strong voltage reference from rotating machines (Short Circuit Ratio >2.0)

• Passive Grid Support: Cannot independently establish or maintain grid voltage and frequency

Technical Characteristics:

• Control Mode: Current source with grid synchronization via Phase-Locked Loop

• Stability Dependency: Performance degrades significantly in weak grid conditions

• Fault Response: Limited fault current contribution (1.2-1.5 pu typically)

• Inertia Provision: None—frequency response depends entirely on external control algorithms

• Black Start Capability: Cannot energize dead systems

Why They Worked—Until Now: Grid-following inverters succeeded because they operated in systems dominated by synchronous generators that provided:

• Strong voltage references for PLL synchronization

• Natural inertia from spinning masses

• Fault current for protection system operation

• Voltage regulation through automatic voltage regulators

Grid-Forming Inverters: The Technology Revolution

How They Work: Grid-forming inverters operate as controlled voltage sources that can independently establish and maintain grid voltage and frequency:

• Autonomous Operation: Create voltage magnitude and frequency without external reference

• Virtual Synchronous Machine (VSM): Emulate rotating machine behavior through sophisticated control algorithms

• Synthetic Inertia: Provide frequency stability faster than mechanical inertia

• Weak Grid Support: Operate effectively with Short Circuit Ratios as low as 1.0

• Black Start Capability: Can energize dead systems and build up grid voltage from zero

Technical Characteristics:

• Control Mode: Voltage source with autonomous frequency and voltage control

• Grid Independence: Performance improves rather than degrades in weak grid conditions

• Fault Current: Programmable fault current contribution up to 2.0 pu

• Inertia Emulation: Software-defined inertia constant (H = 2-10 seconds)

• System Building: Can establish and maintain island operation

The Physics of Synthetic Inertia

Traditional Rotating Machine Inertia

Physical Basis: Synchronous generators provide natural inertia through kinetic energy stored in spinning rotor masses: